Processing metal vapor tubes



Dec. 6, 1966 F. M. JOHNSON PROCESSING METAL VAPOR TUBES Filed Dec. 26,1962 W MK.

m S n M J M d m nite The present invention relates to apparatus for andmethod of processing gas tubes, such as cesium vapor thermionic energyconverters. The invention will be described in connection with suchconverters with the understanding that it can also be used forprocessing other types of gas tubes. The term gas is intended to includemetal vapor.

A thermionic energy converter is an electron tube, having a cathode andanode, for converting heat energy applied to the cathode to electricalenergy in the form of a voltage produced by the tube itself betweencathode and anode terminals. A typical thermionic energy converter tubecomprises a sealed envelope containing a cathode and an anode spacedapart to provide an electron path therebetween. Usually the cathode andanode form parts of, or are in heat transfer relation with, the tubeenvelope, to permit direct heating and cooling, respectively, of theseelectrodes by external means. The materials of the cathode and an-odeare usually chosen such that the effective electron work function of thecathode is substantially higher than that of the anode, to produce aninternal electric field for accelerating electrons from the cathode tothe anode during short circuit condition. Each electron collected by theanode generates heat energy therein, at least equal to the anode workfunction. The output voltage V is a function of the load resistance R.If the load resistance is such that the surface potentials of thecathode and anode are equal and there is no internal voltage drop, theoutput voltage is theoretically equal to the difference in work functionof the cathode and anode, and the output power, VI==R I, is a maximum.In order to approach these optimum conditions, it is necessary toneutralize the space charge of the electrons by use of positive ions.Such ions can be produced in many ways, the best of which is tointroduce an alkali metal vapor, such as cesium, having a low ionizationpotential within the interelectrode space. If the cathode is baretungsten, for example, having an effective work function (4.5 volts)higher than the ionization potential (3.89 volts) of the cesiumoperating at temperatures of the order of 2000 -K., the cesium vaporatoms coming into contact with the hot cathode are ionized by contactionization. In another mode of operation, known as the ball-of-firemode, the cathode has an effective work function lower than theionization potential of the metal vapor used, and the vapor atoms areionized as a result of a complicated process involving successiveexcitations by electron collisions and photons. For example, theoutermost electron in an atom of cesium vapor may be excited by anelectron or photon to a higher energy state, and remain there longenough to be further excited by another electron or photon to the nextenergy 'state, etc., until it is completely removed from the atom toleave a positive cesium ion. Other electrons, after excitation, returnto lower energy states and radiate energy in the form of photons, someof which may in turn produce resonanceexcitation of other cesium atoms.Most of the resonance excitation occurs between the ground state and thelowest lying excited states, 61 and 6P because of their greaterpopulation density. The lifetime of the various excited states are soshort that, for satisfactory operation in the ball-of-fire mode,considerable resonance excitation must be present. Each chain ofphoton-atom energy transfers should involve a large number of such fatesatent transfers before it is terminated by a photon being absorbed bythe tube wall, for example.

The ball-of-fire mode operation is very sensitive to impurity gases inthe converter tube. A converter tube of this type which had deterioratedduring use was found to contain argon gas. Other impurity gases, such asoxygen, are also probably detrimental to satisfactory operation in theball-of-fire mode. It is believed that such impurity gases produce aquenching effect on the excited states of the cesium atoms. For example,the thermal movements of an argon atom, for example, may cause it tocollide with, or pass very close to, a cesium atom and produce a changein the frequency of photons radiated therefrom, and thus prevent, orreduce the probability of, excitation of other atoms by such photons byinterfering with the resonance phenomenon. The most important excitedstates in the ball-of-fire mode of operation are the lowest lyingexcited states, whose atomic lifetimes are of the order of 3 X10" secondfor cesium. The excited states are most sensitive to impurity atoms.

Other detrimental effects of impurity gases in metal vapor tubes, suchas thermionic energy converters, are that they contaminate the surfacesof the cathode and anode, for example, and also increase the rate ofre-combination of positive ions and electrons.

In tubes using low work function cathodes, such as ball-of-fire modeconverters, the electron emitting surface may, for example, bemolybdenum, operating at a temperature of 1500" K., and having anabsorbed partial coating of cesium and an effective workfunction of 2.2volts. The anode may, for example, also be of molybdenum operating at atemperature of 800 K., and having a more complete coating of cesium andan effective work function of 1.7 volts. Any impurity gases in the tubein addition to cesium vapor tend to contaminate the surfaces and changethe effective Work functions thereof. Even in tubes having cathodesoperating at very high temperatures, and consequently no absorbed gas,the anode surfaces are contaminated by impurity gases.

In any electron tube, such as a thermionic energy converter, thatdepends upon a continuous supply of positive ions in the interelectrodespace, to neutralize the space charge of the electrons, for example, itis obviously necessary to keep the rate of re-combination of positiveions and electrons as low as possible. It is believed that impuritygases tend to increase the re-combination rate.

Prior methods of making and processing metal vapor tubes have involvedassembling the tube, evacuating it to a high vacuum, baking the tube toout-gas the parts while continuing the evacuation, sealing off the tubefrom the pump, and then introducing the metal vapor into the tube insome manner. A disadvantage of this method is that the metal vapor whenintroduced attacks the internal surfaces releasing additional gases thatarenot removed but remain as impurities. Moreover, during operation ofthe tube further gases are released, particularly from the hot cathode.

The principal object of the present invention is to provide a method andmeans for effectively removing gases from a sealed electron tubeenvelope without permanently removing a desired ,gas.

Another object of the invention is to provide a new and improved methodof processing a metal vapor tube.

A further object is to provide a novel electron tube having means forremoving impurity gases and means for filling the tube with a desiredgas.

These and other objects are achieved in accordance with one embodimentof the invention by providing a sealed electron tube envelope with amain body portion and at least first and second appendages, condensing adesired gas in the first appendage while heating the 'rest of. the tube,then condensing undesired impurity gases in an end portion of the secondappendage While heating the body portion of the tube to out-gas theinterior surfaces, then sealing-oif the second appendage with theimpurity gases trapped therein, and then evaporating the desired gasfrom the first appendage into the body portion of the tube.

In another embodiment of the invention, a sealed elec tron tube envelopeis' provided with an appendage, in which substantially all the gas inthe envelope is condensed and removed by sealing-ofi the appendage as inthe previously described embodiment, and then a sealed metal vaporcapsule, which may be located in another appendage, is broken byexternal means after the gases are removed to supply metal vapor withinthe envelope.

In the accompanying drawings:

FIG. 1 is a side elevational view, partly in section, of an electrontube of the invention having two empty appendages;

FIG. 2 is a similar view of the tube of FIG. 1 combined with a coldtrap;

FIG. 3 is a similar view showing the step of sealingoff one appendage;

FIG. 4 is a similar view of a modified tube having three appendages;

FIG. 5 is a similar view of a modification in which one appendagecontains a metal vapor capsule; and,

FIG. 6 is an axial sectional view of a typical thermionic energyconverter tube.

FIGS. 1, 2, 3 and 6 show an embodiment of the invention in which athermionic energy converter tube 1 comprises a sealed envelope 3including a main cylindrical body portion 5 and two empty tubularappendages 7 and 9 of different (initial) length sealed at their outerends. Preferably, the appendage 9 is made up of a short section 11 of amalleable metal, such as copper,

sealed to the body portion, and a long section 13 of stainless steelbrazed to the short section 11.

As shown in FIG. 6, the body portion 5 of the envelope 3 may, forexample, comprise two cylindrical or tubular metal sections 15 and 17sealed to a ceramic ring 19 and two metal end plates 21 and 23. In theexample shown, the end plates 21 and 23 include two cup-shaped reentrantportions 21' and 23' which form the cathode and anode, respectively, ofthe tube. The side walls of the portions 21 and 23' may be made verythin and/or of a low heat conducting material to serve as heat dams forthe cathode and anode. The numerals 25 and 27 indicate partial coatingsof alkali metal which are formed on the surfaces of the cathode andanode 21' and 23, respectively, during operation. In the operation ofthe converter tube, the cathode 21' may be heated externally, as by aflame, solar heat, etc., and the anode 23' may be cooled to the desiredtemperature by suitable external means. The operating temperature of thecathode may be any value between 1400 and 2200 K depending on the modeof operation of the tube.

The converter tube 1 is manufactured initially with a small amount ofcesium, or other alkali metal, shown schematically at 29 in FIG. 1,within the envelope 3.

The tube 1 is processed, either immediately after being sealed offduring manufacture or after a period of normal operation, to removetraces of impurity gases from the envelope. In the first step in thisprocessing, the entire tube 1 is heated to-vaporize the cesium, and thenthe shorter appendage 7 is cooled, as by insertion into a water coolingpipe coil 31, while the body portion 5 and longer appendage 9 are kepthot, to condense most of the cesium vapor on the inner wall of theappendage 7. As shown in FIG. 1, the body portion 5 may be heated by anelectric current passed through a coil 33, and the appendage.

9 may be directly heated by connecting the ends thereof to a voltagesource 35. The temperatures are not critical except that the coolingtemperature should not be appreciably lower than the condensingtemperature of cesium, to avoid condensing other gases or vapors oflower condensing temperatures. For example, the appendage '7 may becooled to about 290 K. and the rest of the tube may be heated to about400 K., or higher.

After the cesium vapor has been condensed in appendage 7, the source 35is disconnected and the longer appendage 9 is partially immersed in acold trap 37, as shown in FIG. 2, to condense other gases contained inthe envelope into the immersed end portion of the appendage. During thisstep, the appendage 7 is kept cool, to continue trapping the cesiumtherein, and the body portion 5 is heated to at least 600 K. and thecathode 21' and anode 23' are heated to temperatures above their normaloperating temperatures, to out-gas the body portion 5 and theelectrodes. The cold trap 37 may contain (1) liquid nitrogen (about 77K.), or (2) liquid helium (about 4 K.), depending on the degree ofevacuation desired. In order to remove all gases including helium, itwould be necessary to use a closed liquid helium trap and pump theliquid helium at least to its lambda point, about 2 K. A liquid nitrogentrap is adequate to remove some of the impurity gases that aredetrimental in thermionic energy converter tubes.

As soon as the impurity gases have been condensed in appendage 9, whichrequires only about fifteen minutes, the appendage 9 is sealed-off, asby cold pinch-off of the copper section 11, as shown in FIG. 3, thuspermanently removing the outer end of the appendage with the condensedimpurity gases trapped therein. The tube, thus purified by the removalof impurity gases, can now be used or operated without furtherprocessing, since the envelope including the appendage 7 containing thetrapped cesium will normally become hot enough to vaporize the cesium.On the other hand, the entire envelope can be heated (after removing thecooling coil 31). to a temperature of at least 425 K., as a continuationof the processing described above, to restore the cesium vapor withinthe envelope.

If the appendage 9 in FIG. 1 were made long enough, the processdescribed above could be repeated, to process the tube more than once;but this would ordinarily be impractical because the cold trap may be asmuch as 35 inches long. Instead, the tube can be provided with one ormore additional trap appendages for this purpose. FIG. 4 shows a tube 41having a body portion 5 and two appendages 7 and 9, as in FIGS. 1-3,plus another long appendage 43, to permit two successive processingoperations.

FIG. 5 shows a converter tube 51 which may be identical with the tube 1in FIGS. 1-3 and 6 except for the fact that appendage 7 contains asealed metal capsule containing purified cesium, or other alkali metal,and the envelope 3 may or may not contain free cesium initially.

The tube 51 is processed in accordance with the invention in thefollowing manner. First, the entire tube envelope is heated to atemperature suflicient to vaporize any free cesium contained therein andthe end of appendage 9 is immersed in a cold trap, similar to trap 37 inFIG. 2. The body portion 5 and appendage 7 are then heated to at least600 K. and the cathode and anode of the tube are heated to temperaturesabove their normal operating temperatures, to out-gas the envelope andelectrodes, and then the appendage 9 is sealed-01f as in FIG. 3. Next,the capsule 53 is broken, as by mechanically deforming and then heatingthe appendage 7, to release fresh cesium into the envelope. The entireenvelope is subsequently heated to the desired temperature in order toobtain the necessary cesium pressure. For example, in operation in theball-of-fire mode, the cesium vapor pressure may be from 1 to 10 mm. ofHg.

Other alkali metals can be used in practicing the invention, but cesiumis preferred because it has the lowest ionization potential and workfunction.

What is claimed is:

1. A method of processing a sealed electron tube envelope containing adesired gas and an undesired gas having a lower condensing temperaturethan said desired gas and comprising a main'body portion and twoappendages, comprising the steps of:

(a) selectively condensing most of said desired gas in one of saidappendages at a temperature at least as low as the condensingtemperature of said desired gas and higher than the condensingtemperature of said undesired gas; and i (b) condensing most of saidundesired gas in the other appendage only at a temperature lower thansaid predetermined temperature.

2. A method of processing a sealed electron tube envelope containing aquantity of desired gas which will condense within said envelope at agiven temperature and an undesired gas having a condensing temperaturelower than said given temperature, and comprising a main body portionand two appendages, comprising the steps of:

(a) selectively condensing most of said desired gas in one of saidappendages at a temperature between said given and said lower condensingtemperatures;

(b) then condensing most of said undesired gas in an end portion of theother appendage temperature at least as low as said lower condensingtemperature; and

(c) then sealing-01f said other appendage between said end portion andsaid body portion.

3. A method of processing a sealed electron tube envelope containing aquantity of desired gas and a quantity of an undesired gas having alower condensing temperature than said desired gas, and comprising amain body portion and two appendages, comprising the steps of:

(a) selectively condensing most of said desired gas in one of saidappendages at a temperature at least as low as the condensingtemperature of said desired gas, but higher than the condensingtemperature of said undesired gas;

(b) then condensing most of said undesired gas in an end portion of theother appendage at a temperature at least as low as the condensingtemperature of said undesired gas;

(c) then sealing-off said other appendage between said end portion andsaid body portion; and

((1) then evaporating said desired gas from said one appendage into saidbody portion.

4. A method of selectively removing impurity gases from a sealedelectron tube containing a vaporizable metal and an impurity gas andcomprising a main body portion and two appendages, comprising the stepsof:

(a) heating said envelope to vaporize said metal;

(b) condensing most of the metal vapor in one of said appendages at apredetermined temperature; and

(c) condensing most of said impurity gas in the other appendage at atemperature lower than said predetermined temperature.

5. A method of selectively removing impurity gases from a sealedelectron tube envelope containing a vaporizable metal having arelatively high condensing temperature and an impurity gas having arelatively low condensing temperature and comprising a main body portionand two appendages, comprising the steps of:

(a) heating said envelope to vaporize said metal, and condensing most ofthe metal vapor in one of said appendages at a temperature intermediatesaid relatively high and said relatively low temperatures;

(b) then condensing most of said impurity gas in an end portion of theother appendage at a temperature at least as low as said relatively lowtemperature; and

(c) then sealing-off said other appendage between said end portion andsaid body portion.

6. A method of selectively removing impurity gases from a sealedelectron tube envelope containing a vaporizable metal and an impuritygas and comprising a main body portion and two appendages, comprisingthe steps of:

(a) heating said envelope to vaporize said metal, and cooling a portionof one of said appendages to a temperature at which most of the metalvapor is condensed therein;

(b) then cooling an end portion of the other of said appendages to atemperature at Which most of said impurity gas is condensed therein; and

(c) then sealing-01f said other appendage between said end portion andsaid body portion.

7. A method of selectively removing impurity gases from a sealedelectron tube envelope containing a vaporizable metal and an impuritygas and comprising a main body portion and two appendages, comprisingthe steps of:

(a) heating said envelope to vaporized said metal, and cooling a portionof one of said appendages to a temperature at which most of the metalvapor is condensed therein;

(b) then cooling an end portion of the other of said appendages to atemperature at which most of said impurity gas is condensed therein;

(c) then sealing-01f said other appendage between end portion and saidbody portion; and

(d) then vaporizing said metal vapor from said one appendage into saidbody portion.

8. A method of selectively removing impurity gases from a sealedelectron tube envelope containing a vaporizable metal and an impuritygas and comprising a main body portion and two appendages, comprisingthe steps:

(a) heating said envelope to vaporize said metal, and cooling a portionof one of said appendages to a temperature at which most of the metalvapor is condensed therein;

(b) then heating said body portion to a temperature of at least 600 K.to de-gas the same, while cooling an end portion of the other of saidappendages to a temperature at which most of said impurity gas iscondensed therein; and

(c) then sealing-off said other appendage between said end portion andsaid body portion.

9. A method of selectively removing impurity gases from a sealedelectron tube envelope containing cesium and an impurity gas andcomprising a main body portion containing a plurality of electrodes andtwo tubular appendages, comprising the steps of:

(a) heating said envelope to a temperature of about 400 K. to vaporizesaid cesium;

(b) then cooling a portion of one of said appendages to a temperature ofabout 290 K. to condense most of the cesium vapor therein, whilemaintaining said body portion and other appendage at said first-namedtemperature;

(c) then cooling an end portion of the other of said appendages to atemperature between 2 and 77 K. to condense most of said impurity gasestherein, while heating said body portion to a temperature of at least600 K. and said electrodes to temperatures slightly above their normaloperating temperatures to de-gas the same, and maintaining saidfirst-named portion at said second-named temperature;

(d) then sealing-off said other appendage between said end portion andsaid body portion; and

(e) then heating said one appendage to a temperature of at least 425 K.to vaporize cesium into said body portion.

10. A method of selectively removing impurity gases from a sealedelectron tube envelope containing cesium and impurity gases andcomprising a main body portion and two appendages, comprising the stepsof:

(a) heating said envelope to vaporize said cesium, and

cooling a portion of one of said appendages to a said mersing an endportion of the other of said append- 5 ages in a liquid helium trap tocondense most of said impurity gases in said end portion; and

(c) then sealing-off said other appendage above the liquid level in saidtrap.

11. A method of processing a sealed electron tube envelope containing asealed metal vapor capsule and impurity gases and comprising a main bodyportion and an appendage, comprising the steps of:

(a) heating said envelope to de-gas the same, and condensing most of thesaid impurity gases within an end portion of said appendage;

(b) then sealing ofl? said appendage between said end portion and saidbody portion; and

(c) then releasing metal vapor from said capsule into said envelope.

12. A method of processing a sealed electron tube envelope containingimpurity gases and comprising a main body portion and two appendages,one of which contains a sealed cesium capsule, comprising the steps of:

(a) heating said envelope to de-gas the same, and condensing most ofsaid impurity gases Within an end portion of the other of saidappendages;

(b) then sealing-off said other appendage between said end portion andsaid body portion; and

(c) then releasing cesium vapor from said capsule into said bodyportion.

References Cited by the Examiner UNITED STATES PATENTS 1,529,597 3/1925Langmuir 316-10 2,277,691 3 1942 Curwen 316-21 2,313,788 3/1943 Van Dyke316-21 2,456,968 12/ 1948 Longini 316-24 3,002,116 9/1961 Fisher 31043,056,912 10/1962 Forman 310-4 20 FRANK E. BAILEY, Primary Examiner.

MILTON O. HIRSHFIELD, Examiner.

J. W. GIBBS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,290,llO December 6, 1966 Fred M. Johnson It is hereby certified that errorappears in the above numbered patant requiring correction and that thesaid Letters Patent should read as corrected below.

Column 5, line 25 after "appendage" insert at a Signed and sealed this19th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A METHOD OF PROCESSING A SEALED ELECTRON TUBE ENVELOPE CONTAINING A DESIRED GAS AND AN UNDESIRED GAS HAVING A LOWER CONDENSING TEMPERATURE THAN SAID DESIRED GAS AND COMPRISING A MAIN BODY PORTION AND TWO APPENDAGES, COMPRISING THE STEPS OF: (A) SELECTIVELY CONDENSING MOST OF SAID DESIRED GAS IN ONE OF SAID APPENDAGES AT A TEMPERATURE AT LEAST AS LOW AS THE CONDENSING TEMPERATURE OF SAID DESIRED GAS AND HIGHER THAN THE CONDENSING TEMPERATURE OF SAID UNDESIRED GAS; AND (B) CONDENSING MOST OF SAID UNDESIRED GAS IN THE OTHER APPENDAGE ONLY AT A TEMPERATURE LOWER THAN SAID PREDETERMINED TEMPERATURE. 